I know a best practice is to replace connecting rod bolts. The only new ones I could find lack the flange on the originals.

The flange serves to lock the nut in place, by punching it down into the groove on the rod. What's the best way to lock these new nuts?

I never replace T4 nuts and bolts for stock-ish builds, it's not a problem area.
To lock the nut I use green locktite 640.

BTW this is for stock T1 and 4, I use the reccomended lube for ARP and Carrillo bolt/nuts on performance builds.

Thanks, Mark. Why green and not high-temp red?

Something that has worked for me for 30 years. Way more holding power than red loctite.